On the post editing page, check the box that says "Edit timestamp", then set the date and time to be whatever you want, past or future.

I don't see where the "Edit timestamp" is on the posting page found here. http://www.creativewebsights.com/b2evolution/blogs/admin/

I can't get to your posting page without logging in.

At the bottom of the posting textarea are the buttons that say Preview, Save, Spellcheck, etc. Right below that is a box called "Advanced properties," and the first item at the top of that box should be "Edit Timestamp." If you don't see it then it could be a permissions problem. Are you in the administrator group with a user level of 10?

I figured it out. I went into the Admin controls and gave my blog name a higher level and then it appears.